The present invention relates to an active matrix liquid crystal display apparatus and a manufacturing method thereof.
In an active matrix liquid crystal display apparatus, a method in which a direction of an electric field applied to the liquid crystal is parallel with a substrate (hereinafter, "lateral direction electric field method") is used mainly as a method of obtaining a wide viewing angle (for example, Japanese Unexamined Patent Publication No. 254712/1996). When adopting this method, it is clear that a change in contrast when the viewing angle direction is changed and inversion of gray scale level can be small (for example, M.Oh-e and the others, Asia Display '95.pp.577-580).
FIG. 9 schematically shows an arrangement of one pixel of a substrate of a TFT (thin film transistor) array which is an essential element of a conventional active matrix liquid crystal display apparatus using this method. An image signal is supplied from a signal line 2 to a pixel electrode 6 via a TFT 4 switched by a gate line 1, and an electric field whose direction is parallel with the substrate is formed between the pixel electrode 6 and a counter electrode 5 so that liquid crystal is driven. The counter electrode 5 is connected with a common line 3. The substrate of the TFT integrate apparatus is composed of a pixel section 7, the pixel of which are arranged in a matrix form, and a terminal section 8 for inputting a signal from a circuit (FIG. 10). A counter substrate is laminated onto the pixel section 7 with the liquid crystal being sandwiched therebetween, and a circuit for transmitting an image signal to the gate line and the signal line is mounted to the terminal section 8 so that the liquid crystal display apparatus is manufactured.
The following will describe a method of manufacturing the substrate of the TFT array which is a component of the active matrix liquid crystal display apparatus on reference to a sectional view showing the process in FIG. 11(a) to 11(d). The counter electrode 5 and the common line 3 are formed on a glass substrate 10 simultaneously with the gate line 1 (FIG. 11(a)). The gate line 1 serves also as a gate electrode of the TFT. Next, after a gate insulating film 11 is deposited on the whole surface, amorphous silicon 12 and amorphous silicon 13 with which impurity was doped are formed (FIG. 11(b)). The signal line 2 and the pixel electrode 6 are formed at the same time when a source/drain area 14 of the TFT is formed. Thereafter, the amorphous silicon 13 with which impurity was doped is removed by dry etching or the like using the source/drain area as a mask (FIG. 11(c)). Finally, a passivation film 9 is formed on the whole surface by a transparent insulating film made of silicon nitride, silicon oxide or the like (FIG. 11(d)). The respective layers are formed by the processes of deposition, photolithography and etching. The photolithography process is a method such that coating, exposure and development of a photoresist are carried out so that the photoresist is formed into a desired shape. The exposure is the core process of the all, and in the manufacturing of the active matrix liquid crystal display apparatus, one of the stepper method and the mirror projection method is mainly adopted. In the stepper method, the liquid crystal display apparatus is divided into two or more areas, and while the stage is being moved, the exposure is carried out with a mask being exchanged in every area. In the mirror projection method, the liquid crystal display apparatus is not divided, and one large mask and a glass substrate are scanned integrally and the exposure is carried out collectively. In the stepper method, since accuracy of lamination between the respective layers is high in the whole area of the screen, the characteristic, capacity and the like of the TFT become uniform in the plane, so a DC voltage component which is generated due to the non-uniformity of the characteristic and capacity in the plane can be small, and thus there is an advantage that the liquid crystal display apparatus, in which a liquid crystal material is not easily deteriorated and which has high reliability, can be manufactured. Meanwhile, in the mirror projection method, since the exposure is carried out collectively, there is an advantage such that that throughput can be improved. FIG. 3 shows a conventional divisional exposing method in the case of using the stepper method. The pixel section 7 and the terminal section 8 shown in FIG. 10 are divided into some areas (in the drawing, divided into four areas), and the areas are exposed respectively by using different masks.
In the case where a liquid crystal display apparatus using the lateral direction electric field method is manufactured by the stepper method, as mentioned above, the liquid crystal display apparatus with high reliability can be manufactured, but there arises a problem that a boundary where the divisional exposure is carried out is detected as display unevenness. Also in a liquid crystal display apparatus adopting the TN method using a longitudinal direction electric field, a boundary is occasionally detected as display unevenness in a portion where the lamination between the respective layers is shifted greatly, but in the lateral direction electric field method, a boundary is detected as display unevenness even when the lamination is not shifted, and the boundary is detected more easily. It is an object of the present invention to provide a liquid crystal display apparatus, in which accuracy in a width of a comb-shaped electrode is improved in the plane (particularly in a boundary of divisional exposure), and in a lateral direction electric field method, display unevenness in the boundary portion of the divisional exposure is reduced and simultaneously high reliability is obtained, and to provide a manufacturing method thereof.
FIG. 12 is a result of obtaining a relationship between a changing amount of an electrode width and a changing rate of luminance according to an experiment. As a result, it was clear that in the lateral direction electric field method, a change in the electrode width caused a change in luminance. From FIG. 12, it is found that in order to suppress the changing rate of luminance to not more than 6%, for example, scattering of the electrode width (permissible value of the electrode width) should be suppressed to not more than about 0.3 .mu.m. In this case, the electrode width is 9 .mu.m. When the changing rate of luminance in the boundary is not less than about 6%, the boundary portion of divisional exposure is detected clearly. Therefore, in this case, it is necessary to suppress a difference between the electrode widths in the boundary of the divisional exposure to not more than about 0.3 .mu.m in the liquid crystal display apparatus using the lateral direction electric field method. The intensity of the lateral direction electric field is considered to be proportional to the electrode width, and the above-mentioned permissible value is also considered to be proportional to the electrode width. Therefore, it is found that the following (please see formula (1)) relation between a limit value (.DELTA.W) of the permissible value of the electrode width and a design value (S) of the electrode width in the boundary of divisional exposure should be satisfied. EQU .DELTA.W&lt;0.3.times.S/9=S/30 (1)
Also in conventional TFT-LCD using the TN method, a change in luminance in a boundary of exposure is a problem, and it is caused mainly by parasitic capacity of TFT or the like. As a countermeasure against this problem, as disclosed in Japanese Unexamined Patent Publication No. 305651/1992, accuracy of lamination is improved, and storage capacitance is increased, a boundary is made to be zigzag so as to be unclear, and a boundary of exposure between a gate electrode and a source/drain electrode are in different positions.